Editing Semi-trailer truck (section)

===North America===
[[File:FEMA - 13788 - Photograph by Mark Wolfe taken on 07-11-2005 in Alabama.jpg|thumb|right|Various American trucks lined up]]
[[File:HuckepackCrop.JPG|thumb|Tractor unit hauling tractor units in [[Idaho]]]]

In [[North America]], the combination vehicles made up of a powered semi-tractor and one or more semitrailers are known as "semis", "semitrailers",<ref>{{cite web| url = http://www.merriam-webster.com/dictionary/semitrailer| title = Merriam-Webster| date = 28 May 2023}}</ref> "tractor-trailers", "big rigs", "semi-trucks", "eighteen-wheelers", or "semi-tractor-trailers".{{CN|date=October 2024}}

The [[tractor unit]] typically has two or three [[axle]]s; those built for hauling heavy-duty commercial-construction machinery may have as many as five, some often being [[Axle#Lift axle|lift axles]].

The most common tractor-cab layout has a forward engine, one steering axle, and two [[drive axle]]s. The fifth-wheel trailer coupling on most tractor trucks is movable fore and aft, to allow adjustment in the weight distribution over its rear axle(s).{{CN|date=October 2024}}

Ubiquitous in Europe but less common in North America since the 1990s, is the [[Cab over|cabover engine]] configuration, where the driver sits next to or over the engine. With changes in the US to the maximum length of the combined vehicle, the cabover was largely phased out of North American over-the-road (long-haul) service by 2007. Cabovers were difficult to service; for a long time, the cab could not be lifted on its hinges to a full 90-degree forward tilt, severely limiting access to the front of the engine.{{CN|date=October 2024}}

{{As of|2016|5|25}}, a truck could cost {{US$|100,000}}, while the diesel fuel cost could be $70,000 per year.<ref>{{cite web |url=http://askthetrucker.com/what-it-really-costs-to-own-a-commercial-truck/ |title=What It Really Costs to Own a Commercial Truck |first=Allen |last=Smith |date=25 May 2016 |publisher=Ask The Trucker |access-date=18 May 2017 |quote=pay over $100,000 for your first commercial vehicle. 18-wheeler drinks .. easily more than $70,000 annually}}</ref> Trucks average from {{convert|4|to|8|mpgus|L/100 km}}, with fuel economy standards requiring better than {{convert|7|mpgus|L/100km}} efficiency by 2014.<ref>{{cite web |last=Dyer |first=Ezra |url=http://www.popularmechanics.com/cars/news/pictures/10-things-you-didnt-know-about-semi-trucks-5#slide-5 |title=10 Things You Didn't Know About Semi Trucks |publisher=Popularmechanics.com |access-date=25 March 2016}}</ref> Power requirements in standard conditions are {{cvt|170|hp}} at {{cvt|55|mph|0}} or {{cvt|280|hp}} at {{cvt|70|mph|0}}<!--plus engine management-->, and somewhat different power usage in other conditions.<ref>{{cite web |url=http://electracold.com/CAT.pdf |title=Understanding Tractor-trailer Performance |publisher=[[Caterpillar Inc.|Caterpillar]] |page=5 |date=2006 |id=LEGT6380}}</ref>

[[File:RoadtrainCrop.JPG|thumb|Rocky Mountain Double]]

The [[semi-trailer|cargo trailer]] usually has [[tandem]] axles at the rear, each of which has [[Glossary of trucking industry terms in the United States#Dual wheels|dual wheels]], or eight tires on the trailer, four per axle. In the US it is common to refer to the number of wheel hubs, rather than the number of tires; an axle can have either single or dual tires with no legal difference.<ref>{{cite web |url=http://www.rsa.ie/Documents/Vehicle%20Std%20Leg/Vehicle%20regs/Weights_Dimensions_Leaflet.pdf |title=Guidelines on Maximum Weights and Dimensions |publisher=Ireland Road Safety Authority |date=February 2013 |access-date=26 June 2014}}</ref><ref>{{cite book |last=Crismon |first=Fred W |title=US Military Wheeled Vehicles |edition=3 |year=2001 |publisher=Victory WWII Pub. |isbn=0-970056-71-0 |page=10}}</ref> The combination of eight tires on the trailer and ten tires on the tractor is what led to the [[moniker]] ''eighteen wheeler'', although this term is considered by some truckers to be a [[misnomer]] (the term "eighteen-wheeler" is a nickname for a five-axle over-the-road combination). Many trailers are equipped with movable [[Tandem#Trucks|tandem axles]] to allow adjusting the weight distribution.{{CN|date=October 2024}}

To connect the second of a set of doubles to the first trailer, and to support the front half of the second trailer, a converter gear known as a "dolly" is used. This has one or two axles, a fifth-wheel coupling for the rear trailer, and a tongue with a ring-hitch coupling for the forward trailer. Individual states may further allow longer vehicles, known as "longer combination vehicles" ([[road train|LCVs]]), and may allow them to operate on roads other than Interstates.{{CN|date=October 2024}}

[[Long combination vehicle]] types include:
[[File:UPS Freight truck, Motor Cargo & Overnite trailers.jpg|thumbnail|right|[[STAA doubles|STAA double]] pup {{cvt|28.5|ft}} trailers ]]
* Doubles (officially "[[STAA doubles]]", known colloquially as "a set of joints"): Two {{convert|28.5|ft|m|1|abbr=on}} trailers.
* B-Doubles: Twin {{convert|33|ft|m|1|abbr=on}} trailers in B-double configuration (very common in Canada but rarely used in the United States).
[[File:UPS Truck in Beatty Nevada (24.06.2009).jpg|thumb|UPS truck with triples in [[Nevada]]]]
* Triples: Three {{cvt|28.5|ft|1}} trailers.
[[File:Road train (27).jpg|thumb|{{cvt|53|ft}} container turnpike double in [[Canada]]]]
* Turnpike Doubles: Two {{cvt|40|ft|1}} – {{cvt|53|ft|1}} trailers.
* Rocky Mountain Doubles: One {{cvt|40|to|53|ft|1}} trailer (though usually no more than {{cvt|48|ft|1}}) and one {{cvt|28.5|ft|1}} trailer (known as a "[[road train#Dog-trailer (dog trailer)|pup]]").
* In Canada, a Turnpike Double is two {{convert|53|ft|m|1|abbr=on}} trailers, and a Rocky Mountain Double is a {{convert|50|ft|m|1|abbr=on}} trailer with a {{convert|24|ft|m|1|abbr=on}} "pup".<ref>{{cite web| url = http://www.bclaws.ca/civix/document/id/complete/statreg/30_78#section7.08| title = British Columbia, max. dimension of semi-trailer truck}}</ref><ref>{{cite web| url = http://www.manitoba.ca/mit/mcd/resources/pdf/mb_vehicle_weights_and_dimensions_guide.pdf| title = Manitoba, max. dimension of semi-trailer truck}}</ref><ref>{{cite web| url = https://cdn.ymaws.com/www.ctea.ca/resource/resmgr/Docs/2016_VW&D_in_Ontario_Public_.pdf| title = Ontario, max. dimension of semi-trailer truck}}</ref>

The US federal government, which only regulates the [[National Highway System (United States)|Interstate Highway System]], does not set maximum length requirements (except on auto and boat transporters), only minimums. Tractors can pull two or three trailers if the combination is legal in that state. Weight maximums are {{convert|20000|lb|kg|abbr=on}} on a single axle, {{convert|34000|lb|kg|abbr=on}} on a tandem, and {{convert|80000|lb|kg|abbr=on}} total for any vehicle or combination. There is a maximum width of {{convert|8.5|ft|m|abbr=on}} and no maximum height.<ref>{{cite web |url=https://ops.fhwa.dot.gov/Freight/sw/overview/index.htm |title=Commercial Vehicle Size and Weight Program |publisher=US Department of Transportation |series=Freight Management and Operations |date=May 2003 |access-date=7 August 2016 |id=FHWA-OP-03-099}}</ref><ref>{{cite web |url=https://ops.fhwa.dot.gov/freight/publications/size_regs_final_rpt/index.htm#cmv |title=Federal Size Regulations for Commercial Motor Vehicles |publisher=US Department of Transportation |access-date=2 October 2012}}</ref>

Roads other than Interstates are regulated by individual states, and laws vary widely. Maximum weight varies between {{convert|80,000|lb|kg|abbr=on}} to {{convert|171,000|lb|kg|abbr=on}}, depending on the combination.<ref>{{cite web |url=http://www.bigtruckguide.com/heaviest-semi-trucks-in-the-united-states-and-canada/ |title=Top 10 Heaviest Semi Trucks in the United States and Canada |website=BigTruckGuide.com |first=Paul |last=Jakubicek |date=16 January 2015 |access-date=12 July 2016}}</ref> Most states restrict operation of larger tandem trailer setups such as triple units, turnpike doubles, and Rocky Mountain doubles. Reasons for limiting the legal trailer configurations include safety concerns and the impracticality of designing and constructing roads that can accommodate the larger [[wheelbase]] of these vehicles and the larger minimum turning radii associated with them. In general, these configurations are restricted to the Interstates. Except for these units, double setups are not restricted to certain roads any more than a single setup. They are also not restricted by weather conditions or "difficulty of operation". The Canadian province of Ontario, however, does have weather-related operating restrictions for larger tandem trailer setups.<ref>{{cite web |url=http://www.mto.gov.on.ca/english/trucks/lcv/program-conditions/index.shtml |title=Long Combination Vehicles |access-date=1 June 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130517124815/http://www.mto.gov.on.ca/english/trucks/lcv/program-conditions/index.shtml |archive-date=17 May 2013 }}</ref>